1. Field of the Invention
The present invention relates, generally, to methods for producing silicon carbide and, more particularly to methods for producing silicon carbide particles using solar thermal energy. Specifically, the present invention relates to improved methods which are more efficient than previously available for producing silicon carbide particles.
2. Description of the Prior Art
Silicon carbide is an abrasive material which is widely used in a variety of grinding and polishing applications. Its hardness is 9.5 on the Mohs scale and therefore is only slightly less than the hardness of diamond (which is 10 on the same scale).
Silicon carbide is made by reacting silica with carbon at 1800-1900.degree. C. The chemical reaction for the process is: EQU SiO.sub.2 +3C.fwdarw.SiC+2CO
Silicon carbide was first made by this reaction in 1891 in an electric arc furnace. At that time the silicon carbide was used by jewelers as a polishing agent. Today silicon carbide powders are produced in various grades. The differences between the high and low grades are purity and particle size. Low grade silicon carbide consists of relatively low purity (less than 98%) particles having a diameter of 0.1-10 millimeters. High grade silicon carbide consists of high purity particles (greater than 98%) having a diameter of 0.5 to 5 microns. Low grade silicon carbide is widely used as an abrasive in polishing and grinding wheels.
Because of its high melting point (2200.degree. C.), it is also used as refractory in high temperature furnaces. High grade silicon carbide is also sintered to make various types of components, such as fittings, feed throughs, etc. for high temperature applications. Worldwide production of silicon carbide is about 200,000 tons per year.
Conventional manufacturing methods for producing silicon carbide utilize electric-resistance furnaces to provide the thermal energy which is necessary to produce the material. Such methods use a significant amount of energy.
The conventional electric resistance furnace typically consists of a bed (10 feet by 10 feet by 30 feet) of well mixed silica and carbon particles. Two solid carbon electrodes extend into the bed at both ends. The electrodes are connected with a core of pure carbon particles which carry the electrical current when the reaction is initiated. The bed is heated with electricity for a period of 8 hours or more during which the process continuously draws 6000 amps of current at 230 volts. The reaction between the silica and carbon is virtually instantaneous once 1800.degree. C. is reached. Heat transfer limitations within the bed require the long period of heating to allow the large mass of material to reach 1800.degree. C. The heating period is followed by a cooling period of 20 hours. The bed produces silicon carbide ingots that range in size from 1 foot in diameter down to diameters of approximately 100 microns. The reacted material is manually fed to a crusher to reduce the size of the ingots to small particles. The particles are then chemically treated with acid to remove unreacted silica along with contaminants from the crushing process. Excess carbon is removed by oxidation at temperatures less than 500.degree. C. The particles are then screened and classified according to particle size and shape.
The costs associated with handling large quantities of electricity, and the costs associated with crushing the silicon carbide to smaller particles, account for the major costs in producing the material using conventional techniques.
U.S. Pat. No. 4,419,336 (Kuriakose) describes an improved electric resistance furnace for producing silicon carbide. U.S. Pat. No. 4,534,948 (Baney) describes a process for producing silicon carbide using specific polysilane polymers as starting materials. The polymer is heated to 1600.degree. C. in an inert atmosphere to form silicon carbide. The main advantage of this approach is that the polymer can be pre-formed into fibers or other shapes which the silicon carbide assumes when it forms. These patents do not describe a process for forming silicon carbide using a solar furnace.
U.S. Pat. No. 2,987,961 (Cotton et al.) describes a solar furnace and improved concentrator for focusing solar rays. There is no suggestion in such patent of a process for forming silicon carbide.